The present invention relates to pre-expanded particles of a polyolefin resin and a process for preparing the same. More particularly, the present invention relates to polyolefin resin pre-expanded particles suitable for use in an in-mold foaming method wherein in-mold foamed articles are prepared by filling polyolefin resin pre-expanded particles in a mold capable of being closed but incapable of being hermetically sealed and heating the particles with a heating medium such as steam to fuse them together, and a process for preparing the pre-expanded particles.
Polyolefin foamed articles are superior in chemical resistance, heat resistance and distortion restoration rate after compression as compared to polystyrene foamed articles. Also, in-mold molding of polyolefin pre-expanded particles has the advantage that foamed articles having a complicated shape can be prepared. For these reasons, polyolefin in-mold foamed articles have been popularly used in various fields, for examples, as cushioning packaging materials, returnable delivery boxes and automobile bumper cores.
If a polyolefin resin is heat-treated at a temperature in the vicinity of its melting point, the crystal structure changes to reveal two fusion peaks when measured by differential scanning calorimetry (DSC). JP-A-59-176336 discloses that polyolefin pre-expanded particles prepared by a method which comprises charging polyolefin resin particles, an aqueous dispersion medium containing a dispersing agent and a dispersing aid, and a blowing agent in a pressure vessel, heating the resulting dispersion with stirring to a prescribed pressure and a prescribed temperature, and releasing the dispersion into a low pressure atmosphere to thereby expand the particles (the method being hereinafter referred to as xe2x80x9cpressure release expansion methodxe2x80x9d), are required to have a crystal structure having two fusion peaks as observed on the DSC curve in obtaining good in-mold foaming products.
Further, it is proposed to improve the moldability of polyolefin pre-expanded particles by defining the heat of fusion of a peak appearing on the higher temperature side of the DSC curve of the pre-expanded particles within a specific range with respect to each kind of resins. For examples, it is disclosed that the moldability is improved when the heat of fusion of the high-temperature-side peak of polyethylene resin pre-expanded particles falls within the range of not less than 5 J/g (JP-A-64-1741), when the heat of fusion of the high-temperature-side peak of polyethylene resin pre-expanded particles falls within the range of 8 to 25.1 J/g (JP-A-2-43206), and when the heat of fusion of the high-temperature-side peak of polypropylene resin pre-expanded particles falls within the range of 15 to 35 J/g (JP-A-5-179049 and JP-A-11-106550).
Like this, in case of conducting in-mold molding of polyolefin pre-expanded particles, it is required to define a parameter showing the crystal state of the resin, e.g., the heat of fusion of the high-temperature-side peak measured by differential scanning calorimetry, and to keep the parameter within a certain range. In practical in-mold foaming, a more preferable range is selected depending on the kind of resin, expansion ratio, molding method and the like. However, despite that the parameter falls within a preferable range, dimensional change of molded articles and molding cycle change occur and, therefore, it is hard to say that the foaming is controlled on a satisfactory level. Dimensional change results in generation of inferior products since the products do not have the required dimensions. Molding cycle change results in lowering of productivity since the production must be conducted at the longest molding cycle in order to avoid generation of inferior products.
It is an object of the present invention is to provide pre-expanded particles of a polyolefin resin which can be molded in a mold without causing molding cycle change to give cellular molded articles having a good dimensional stability.
A further object of the present invention is to provide a process for preparing polyolefin pre-expanded particles which can be subjected to in-mold foaming without causing molding cycle change and dimensional change of foaming products.
These and other objects of the present invention will become apparent from the description hereinafter.
It has now been found that the dimensional change and the molding cycle change are caused by variation in crystal state of pre-expanded particles and the above-mentioned objects can be achieved by minimizing the variation. It has also been found that polyolefin pre-expanded particles having a uniform crystal state can be prepared by carrying out the pressure release expansion method in such a manner that the temperature or pressure inside a pressure vessel is elevated with the lapse of time in the step of releasing a dispersion of resin particles impregnated with a blowing agent from the pressure vessel into a lower pressure zone.
Thus, in accordance with the present invention, there is provided a pre-expanded particle of a polyolefin resin wherein said pre-expanded particle shows two fusion peaks on a DSC curve when measured by differential scanning calorimetry (DSC) and the proportion of the heat of fusion of the peak appearing on the high temperature side based on the total heat of fusion of the two peaks represented by the equation: [Qh/(Ql+Qh)]xc3x97100 falls within the range of 10 to 60% in which Qh is the heat of fusion of the peak appearing on the high temperature side and Ql is the heat of fusion of the peak appearing on the low temperature side, and the standard deviation of said proportion is not more than 1.2.
The present invention also provides a process for preparing pre-expanded particles of a polyolefin resin.
In accordance with an embodiment of the present invention, there is provided a process for preparing pre-expanded particles of a polyolefin resin comprising the steps of charging polyolefin resin particles, an aqueous dispersion medium containing a dispersing agent and optionally a dispersing aid, and a blowing agent in a pressure vessel, heating the resulting dispersion with stirring to a prescribed pressure and a prescribed temperature to impregnate the resin particles with the blowing agent, and releasing the dispersion into a low pressure atmosphere to thereby expand the particles, wherein the inner temperature of the pressure vessel during the release is raised stepwise or continuously at a rate of 0.03 to 0.5xc2x0 C. per 10 minutes.
In accordance with another embodiment of the present invention, there is also provided a process for preparing pre-expanded particles of a polyolefin resin comprising the steps of charging polyolefin resin particles, an aqueous dispersion medium containing a dispersing agent and optionally a dispersing aid, and a blowing agent having an effect of lowering the melting point of the polyolefin resin in a pressure vessel, heating the resulting dispersion with stirring to a prescribed pressure and a prescribed temperature to impregnate the resin particles with the blowing agent, and releasing the dispersion into a low pressure atmosphere to thereby expand the particles, wherein the inner pressure of the pressure vessel during the release is raised stepwise or continuously at a rate of 0.005 to 0.05 MPa per 10 minutes.